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Abstract

We investigate systematically the influence of the nature of thiol-type capping ligands
on the optical and structural properties of highly luminescent CdTe quantum dots synthesized
in aqueous media, comparing mercaptopropionic acid (MPA), thioglycolic acid (TGA),
1-thioglycerol (TGH), and glutathione (GSH). The growth rate, size distribution, and
quantum yield strongly depend on the type of surface ligand used. While TGH binds
too strongly to the nanocrystal surface inhibiting growth, the use of GSH results
in the fastest growth kinetics. TGA and MPA show intermediate growth kinetics, but
MPA yields a much lower initial size distribution than TGA. The obtained fluorescence
quantum yields range from 38% to 73%. XPS studies unambiguously put into evidence
the formation of a CdS shell on the CdTe core due to the thermal decomposition of
the capping ligands. This shell is thicker when GSH is used as ligand, as compared
with TGA ligands.

Keywords:

CdTe; Surface ligands; Optical properties; Semiconductor nanocrystals

Background

Over the past two decades, semiconductor nanocrystals have attracted great attention
of researchers due to their unique optical properties. In particular, luminescent
quantum dots (QDs) are defined as semiconductor structures with physical dimensions
that are smaller than the exciton Bohr radius
[1,2]. These materials exhibit a strong quantum confinement effect, and this effect causes
the appearance of size-dependent optical properties, which has attracted great attention
for application of QDs in different technological areas, including biological labeling,
light-emitting diodes, and photovoltaic devices
[3].

QDs can be produced via a number of synthetic methods. These techniques have great
advances in recent years, which have enabled the synthesis of monodisperse nanocrystals
with good optical properties as well as different compositions and morphologies
[4]. Up to now, the most successful method to prepare highly luminescent II-VI colloidal
semiconductors is the organometallic synthetic route, which uses trioctylphosphine
oxide and trioctylphosphine as surface ligands, in order to avoid nanocrystal growth
and aggregation. Alternatively, organic metal salts such as Cd carboxylates or phosphonates
can be reacted with the chalcogenide source in 1-octadecene. However, these methods
require high temperatures, and the resulting nanoparticles are insoluble in water,
which makes the final product incompatible with the biological systems
[5-7]. More recently, QDs have been prepared in aqueous medium because this synthetic approach
is simpler, less toxic, and generates water-soluble nanocrystals that are directly
biologically compatible
[8]. Nevertheless, this method generally produces nanoparticles with lower fluorescence
quantum yields, when compared to the synthesis in organic media. The lower florescence
quantum yield is attributed to the defects and traps on the surface of the nanocrystals.
Therefore, researchers have investigated the influence of the surface ligands so as
to remove these defects and improve the optical properties
[4,9,10].

Surface ligands consist of a polar anchoring group and either an apolar hydrocarbon
chain (synthesis in organics) or a charged group (synthesis in water). These ligands
must dynamically adsorb on/desorb from the surface of the nanocrystals at the synthesis
temperature in order to allow for growth while the nanoparticles are stabilized against
aggregation. Peng et al. reported on the effect of amine ligands on the growth of CdSe QDs and proved that
the ligand dynamics on the nanocrystal surface depends on the reaction temperature
and on the concentration and chain length of the stabilizers
[11]. Earlier studies showed that ligands play an important role during the formation
of nanocrystals, exerting a strong effect on both the nucleation and growth stages.
Hence, surface ligands can control the size, shape, growth kinetics, and optical properties
of the QDs
[4,10-14].

Also, in the case of water-soluble CdTe nanocrystals, the influence of some types
of surface ligands on the structural and optical properties has been studied
[15,16]. The most often used surface ligands are thioglycolic acid (TGA) or mercaptopropionic
acid (MPA). The growth kinetics of TGA-coated CdTe has been analyzed quantitatively
by means of dynamic light scattering (DLS) measurements, and the growth rates, size
distributions, critical radii, and diffusion constants have been calculated in the
framework of the theoretical Sugimoto model
[17]. There are two distinct regimes of kinetics: (1) slow increase in the hydrodynamic
radius and (2) faster growth of nanoparticles compared with the previous regime. These
two kinetic regimes allow for a certain control of nanocrystal size and size distribution,
with no need for post-preparative fractionation techniques.

In 2010, Lesnyak et al.
[18] described a novel ligand, 5-mercaptomethyltetrazole, for the aqueous synthesis of
CdTe nanocrystals. Tetrazoles are five-membered cyclic compounds containing four nitrogen
atoms of different types (pyrrole and pyridine type). CdTe nanocrystals obtained via
the ‘standard method’ but using mercaptomethyltetrazole instead of TGA as the stabilizer
exhibited fluorescence in the 510- to 610-nm range, depending on the reflux time,
and fluorescence quantum yields reaching up to 60%. Upon addition of a solution of
Cd2+ ions, the CdTe nanocrystals irreversibly formed hydrogels, i.e., highly porous 3D
networks
[18]. The interaction of MPA-capped CdTe nanocrystals, synthesized in aqueous media with
cysteine and homocysteine, has already been described
[19]. Glutathione (GSH), a thiol-containing tripeptide, has been shown to be able to provide
improved biocompatible capping for semiconductor nanocrystals as compared with many
other water-soluble ligands
[20]. Moreover, GSH appears to work best with CdTe in terms of promoting high photoluminescence
[10,21].

In the present work, we conducted a comparative study of the synthesis of CdTe QDs
prepared in aqueous media using four different surface ligands: MPA, TGA, 1-thioglycerol
(TGH), and GSH. The influence of these ligands on the surface of the nanocrystals
was evaluated on the basis of the changes in their optical properties and their sizes.

Syntheses of MPA-, TGA-, TGH- and GSH-coated CdTe QDs

The experimental procedure was performed according to
[22], but different stabilizing ligands were utilized. Briefly, 0.8 mmol of tellurium
powder and 1.6 mmol of sodium borohydride were diluted in 20 mL of Milli-Q water in
a 25-mL three-neck flask. The reaction mixture was heated to 80°C under argon flow
in order to obtain a clear deep red solution. The resulting NaHTe was used as tellurium
source. Next, 0.4 mmol of Cd solution and 1.4 mmol of MPA were mixed in 80 mL of Milli-Q
water, and the pH value was adjusted to 10.0 by the addition of NaOH (1.0 molL−1). This solution was heated at 100°C under argon bubbling and then 4.0 mL of freshly
prepared NaHTe was added with the aid of a syringe. The resulting solution was refluxed
at 100°C for different times in order to obtain CdTe QDs of different sizes. Aliquots
were taken at defined time intervals, and their UV-vis absorption and photoluminescence
(PL) spectra were recorded. Samples were precipitated by addition of acetone and dried
in vacuum prior to characterization by X-ray diffration (XRD), Fourier-transform infrared
spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron
microscopy (TEM). The same procedure was followed for the synthesis involving the
other surface ligands investigated in this work. For acquisition of the TEM images,
the nanocrystals were submitted to a phase-transfer procedure based on the partial
exchange of the MPA stabilized by dodecanethiol (DDT)
[23]. To this end, 1 mL of an aqueous solution of CdTe was placed in a test tube, and
1 mL of 1-dodecanethiol and 2 to 3 mL of acetone were added to this solution. The
test tube was vigorously shaken and heated to the boiling point of acetone. The transfer
of the nanocrystals to the organic phase was detected from the change in the color
of the latter phase.

Characterization

UV-vis absorption and PL spectra were acquired on a Varian Cary 50 spectrophotometer
(Varian Inc., Palo Alto, CA, USA) and Shimadzu RF-5301 PC spectrofluorophotometer
(Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan), respectively. The spectrofluorophotometer
is equipped with a xenon lamp of 150 W. The absorption and fluorescence measurements
were typically performed with 10-mm quartz cuvettes (Shimadzu) using air-saturated
solutions at room temperature. The fluorescence quantum yield (ϕf) of the nanocrystals was estimated by comparing the integrated emission of the QD
samples, obtained at one excitation wavelength, with that of a standard fluorescent
dye, rhodamine 101
[24,25]. We have used the wavelength excitation of 355 nm. Essentially, stock solutions of
the standard and QD samples with similar absorbance (no higher than 0.02) at the same
excitation wavelength can be assumed to be absorbing the same number of photons. Hence,
a simple ratio of the integrated fluorescence intensities of the two solutions (recorded
under identical conditions) yielded the ratio of the quantum yield values. Since the
quantum yield for the standard sample rhodamine 101 is known (ϕf = 1.0 in water
[24,25]), it is trivial to calculate the quantum yield for the QDs. Identical instrument
settings for the sample and standard solutions were carefully checked, and the solvent
adsorption and emission spectra were subtracted from the absorption and emission spectra
of the sample and standard solutions. This was done directly in the software of the
equipment used. In addition, the measurements were repeated for at least three different
concentrations of the sample and the reference dye. Powder X-ray diffraction (XRD)
patterns were recorded on a Shimadzu XRD-6000 using CuKα radiation. FTIR spectra of the materials were obtained by the conventional KBr pellet
technique, in a GXI spectrum Perkin Elmer spectrometer (PerkinElmer, Waltham, MA,
USA), operating between 4,000 and 400 cm−1. A minimum of 32 scans were recorded with a resolution of 2 cm−1. The KBr salt and the nanoparticles were dried for 2 h under 110°C and 40°C, respectively.
Both samples were kept under vacuum until the moment of the analyses. The sizes of
the nanocrystals were determined by DLS measurements using a HeNe source (λ = 632.8 nm) and a photomultiplier as detector. The correlation functions were calculated
by the BI9000-AT correlator board Brookhaven Inst. Co. (Holtsville, NY, USA). A scattering
angle of 30.0° was used during these measurements, and the samples were maintained
in a thermal bath at 25.0°C with a precision of 0.1°C. TEM was performed on a TEM-FEG
JEM 2100F microscope (JEOL Ltd., Akishima, Tokyo, Japan) operating at 200 kV. XPS
was conducted on an ultrahigh vacuum system (base pressure of 3.0 × 10−10 mbar) equipped with a standard non-monochromatic Mg Kα X-ray source (hν = 1,253.6
eV) and a concentric hemispherical electron-energy analyzer (CLAM2, VG Microtech,
East Sussex, UK). The binding energy (BE) scale was calibrated using the carbon peak
from the surface contamination as reference (C 1s at 284.6 eV).

Results and discussion

There are many studies reporting on the synthesis of CdTe QDs, but only few of them
have described the influence of surface ligands on the growth and optical properties
of these nanocrystals. In this work, we investigated the effect of for different thiol
ligands on the growth and optical properties of CdTe nanocrystals prepared under identical
synthesis conditions. The molecular structures of these ligands are schematically
represented in Figure
1.

Figure 1.Schematic representation of the molecular structures of the surface ligands investigated
in this work.

Representative STEM images of the obtained CdTe QDs are displayed in Figure
2. These images show that the nanocrystals have approximately spherical shape and are
well dispersed without aggregation.

Figure
3 illustrates the temporal evolution of the UV-vis absorption and PL spectra of GSH-coated
CdTe QDs. A longer refluxing time systematically shifts the excitonic absorption and
PL emission peaks to longer wavelengths, which is a clear indication of nanocrystal
growth. This behavior is different from that verified for the other investigated ligands.
Figure
4 presents the temporal evolution of the absorption and emission peaks recorded for
the four surface ligands studied here. The growth rate registered for GSH-coated CdTe
QDs is the highest under the same refluxing conditions. We attribute the special behavior
of GSH to its higher steric hindrance and its stronger tendency to thermal decomposition
as compared to the other ligands. Under reflux, the partial hydrolysis of GSH causes
the incorporation of sulfur in the interior and surface of the CdTe nanocrystals,
thereby forming a CdTe/CdS core/shell or a graded structure
[26-29]. The latter induces spectral shifts but also provides better surface passivation
and control over defects, improving the fluorescence quantum yield. As for the other
three ligands, the growth rates of CdTe QDs capped with are significantly lower, suggesting
that they bind more strongly to the surface of the nanocrystal. While the temporal
evolution of the peak positions is similar for all the three ligands, the size of
the initially formed crystallites is smallest for TGH, followed by MPA and TGA.

Figure 4.Temporal evolution. (a) The absorption and (b) the PL peak position of CdTe QDs capped with different surface ligands.

In Figure
5, the PL linewidth of the QDs (full width at half-maximum (FWHM)) is plotted as a
function of the reaction time. As a rule of thumb, the lower the linewidth, the narrower
is the size dispersion. Contrary to the synthesis in organic solvents, no size focusing
is observed
[30]. For all reaction times, MPA-coated CdTe QDs have the narrowest size distribution
followed by GSH-coated ones. In both cases, a broadening of the linewidth is detected
with longer reflux time. Both TGA- and TGH-coated CdTe yield a much broader initial
linewidth. This behavior indicates that these ligands bind too strongly to the surface
of the nanocrystal, avoiding the sharp separation of nucleation and growth under the
reaction conditions employed here. As for TGH, the linewidth becomes slightly narrower
with reaction time, while a further broadening is verified in the case of TGA. Broad
size distributions can be reduced using the post-preparative size-selective precipitation
technique
[22].

Figure
6 depicts the ϕf of the as-synthesized CdTe QDs at different reaction times. The maximal quantum yields
are achieved after 1 h of synthesis (MPA 73%, GSH 49%, and TGH 38%), except for TGA,
which reached the maximum after half an hour (70%). This demonstrates that short-chain
ligands, such as MPA and TGA, generally give rise to high ϕf, which is likely due to a better surface passivation induced by a higher ligand density
[9]. Under prolonged reflux, there is a decrease in ϕf values for all ligands. The fluorescence quantum yield is strongly dependent on the
surface quality. At longer reflux times, unfavorable adsorption-desorption equilibria
associated with the Ostwald ripening phenomenon can lead to incomplete passivation
of surface trap states of the QDs, resulting in lower fluorescence quantum yield
[31]. Rogach et al.
[8] have proposed a correlation between the values of ϕf and Stokes shift as a rapid technique to evaluate the quality of the samples, without
involving the comparison with luminescence standards. In brief, high ϕf samples generally exhibit a lower Stokes shift than low ϕf samples. In the former case, detrapping of carriers from shallow trap levels occurs,
while in the latter case, a broad distribution of trap states favors non-radiative
de-excitation. A comparison of the ϕf values and Stokes shift of CdTe-capped with GSH, TGA, MPA, and TGH at different synthesis
times is summarized in Table
1. For the smaller molecules (TGA, MPA, and TGH), we observe indeed the general trend
that a decrease in the ϕf value is accompanied by an increase of Stokes shift. In particular, CdTe-TGH show
the lowest ϕf values combined with the highest Stokes shifts. On the other side, CdTe QDs capped
with the larger molecule GSH display a different behavior; although the lowest Stokes
shift is observed, only intermediate ϕf values are measured. The lower Stokes shift indicates that deep traps are efficiently
passivated by a thin, in situ-generated ZnS shell on the CdTe core (vide infra). At the same time, ϕf is probably reduced by a variety of non-radiative de-excitation pathways resulting
from the insufficient passivation of (outer) surface states in case of the sterically
demanding GSH ligands.

The XRD patterns of CdTe powders precipitated from aqueous sols of QDs with an excess
of acetone are given in Figure
7. The nanocrystals belong to the cubic zinc-blende structure with diffraction peaks
at 24°, 40°, and 46.7°, which is consistent with the dominant crystal phase of bulk
CdTe
[32]. However, when GSH is used as surface ligand, the diffraction peaks shift to larger
angles toward the peaks of the CdS. As mentioned before, the thermal decomposition
of GSH favors the formation of CdTe/CdS core/shell or graded structures. TGH-coated
CdTe nanocrystals, on the other hand, undergo a smaller shift toward the peak positions
of CdS, which proves that there is smaller tendency for the formation of CdTe/CdS
structures. Finally, TGA and MPA exhibit intermediate peak positions between those
of cubic CdTe and CdS.

Figure
8 presents the FTIR spectra of the nanocrystals stabilized with different ligands.
The main differences of the spectra between the free and bound ligands are marked
with arrows. In the case of free TGA and MPA, the most pronounced IR absorption bands
occur at 3,500 to 3,000 cm−1 (
), 2,950 cm−1 (
), 2,574 cm−1 (
), 1,707 cm−1 (
), 1,222 cm−1 (
), and 680 cm−1 (
). For the bound ligands, the COO− vibrations at 1,562 cm−1 and 1,397 cm−1 are consistent with the fact that at pH 10, the carboxylic acid group is deprotonated
given its pKCOOH value of 3.67. The S-H vibrations (2,574 cm−1) are not detectable in the IR spectra of any of the bound ligands, which is expected
for thiols covalently bound to the surface of nanocrystals.

The hydrodynamic size of the nanocrystals has been measured by dynamic light scattering
[33-35]. This technique allows for the statistical analysis of the fluctuations in the intensity
of the light scattered by particles in solutions. The fluctuations in the intensity
of the scattered light are expected to undergo exponential decay along time, at a
rate Γ given by the product of the diffusion coefficient D and the squared scattering wave vector q, which is defined in terms of the wavelength λ of the source, the refraction index n, and the scattering angle θ:

(1)

The Einstein-Stokes relation is then used for calculation of the hydrodynamic radius
of the dispersed particles from the diffusion coefficient D, the solvent viscosity η, and the thermal energy kBT:

(2)

The intensity correlation function C(q,t) is proportional to the squared dynamic structure factor S(q,t) (
). In the case of bidispersions, S(q,t) becomes the sum of two exponentials corresponding to each species. The measurements
can then be well fitted by an expression given by the sum of a fast decaying exponential
and a slower one. The decay rate Γ is computed from this fitting of the experimental results, and the diffusion coefficient
is calculated through Equation 1. This coefficient can be replaced directly in Equation
2 yielding the hydrodynamic radius Rh.

Using this technique describe above, we obtained the dynamic structure factor, S(q,t), for the MPA- and GSH-coated CdTe QDs (Figure
9). Following the aforementioned steps, the data shown in Figure
9a,b allow for the determination of the hydrodynamic radii of the nanocrystals. We
calculate the hydrodynamic radii of the nanocrystals at different reaction times (Figure
10). The GSH-coated CdTe QDs (Figure
10a) and the MPA-coated CdTe QDs (Figure
10b) have sizes ranging from 2.1 to 5.1 nm and from 2.1 to 11.9 nm, respectively. The
large sizes determined for MPA ligands can be ascribed to aggregation occurring at
extended reflux times. Indeed, here, the reaction with MPA was conducted for more
than 60 h vs. 5 h in the case of GSH. The DLS results also confirm that the growth
rate of GSH-capped CdTe QDs is higher than that of MPA-capped QDs under the same refluxing
conditions. Nanocrystals with sizes of 6 nm were obtained after approximately 5 and
24 h of synthesis for the GSH and MPA ligands, respectively.

Finally, we examined the surface of the TGA- and GSH-coated CdTe QDs using XPS. The
survey XPS spectra of TGA- and GSH-coated CdTe QDs are displayed in Figure
11. The spectra are characterized by the Cd 3d5/2 peaks at 405.0 and 404.8 eV as well as by the Te 3d5/2 peaks at 572.9 and 572.6 eV for TGA- and GSH-coated CdTe QDs, respectively. The BE
values observed for Te 3d5/2 are characteristic of CdTe
[36], but the values obtained for Cd 3d5/2 are closer to the ones typical of CdS (405.3 eV
[36]). The photoemission spectra also reveal the C 1s and O 1s peaks for the TGA- and GSH-coated CdTe QDs. Comparison between the spectra of both
TGA- and GSH-coated CdTe QDs shows that there are higher carbon and oxygen contents
in the spectrum of the sample using TGA. Higher sulfur contents (S 2s and S 2p) can be verified in the spectrum of GSH-coated CdTe QDs, corroborating the hypothesis
that the decomposition of this ligand increases the amount of sulfur in the surface
layers of the QDs. The S 2p peaks are centered at 162.5 and 162.0 eV in the spectra of TGA- and GSH-coated CdTe,
respectively, consistent with the formation of CdS
[4,37]. Thus, on the basis of the data presented above, we can infer that the decomposition
of GSH leads to the formation of a CdS shell on the CdTe core. The higher amounts
of cadmium and sulfur in the spectrum of GSH-coated CdTe indicate the formation of
a thicker CdS shell than in the case of TGA-coated CdTe. Similar results have been
reported for CdTe/CdS core/shell QDs synthesized using tiopronin and thioacetamide,
where evidence for the formation of the core/shell structure was obtained from the
blueshift of the S 2p peak by 0.4 eV from CdTe to CdTe/CdS
[4].

Conclusions

In this work, we carried out a systematic investigation of different thiol-stabilizing
ligands on the properties of CdTe QDs synthesized in aqueous solution. The growth
rate, size distribution, and quantum yield strongly depend on the type of surface
ligand. Under the same refluxing conditions, the highest growth rate is obtained in
the presence of the GSH ligand. However, TGH ligands bind to the QD surface too strongly,
hindering particle growth and yielding broad size distribution. TGA and MPA ligands
furnish comparable results due to their similar molecular structures. The exception
is the size dispersion at short reaction times, which is much broader for TGA. No
size focusing is observed in any case. With the exception of TGH-coated CdTe QDs,
for which a slight decrease in the PL linewidth is observed with reflux time, the
QDs exhibit broader size distributions and lower quantum yields for prolonged reaction
times. The infrared spectra indicate that the ligands are connected to the nanocrystal
surface via the SH group. XPS results clearly evidence the formation of a CdS shell
on the CdTe core due to the thermal decomposition of the surface ligands. This CdS
shell is thicker when GSH is used as the capping ligand as compared with TGA. In most
cases, the shell thickness is directly correlated to the photostability of the QDs,
and therefore, GSH-coated CdTe QDs are the most promising candidates for applications
relying on good photoluminescence properties such as biological labeling or displays
and lighting.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

FOS and MSC carried out the preparation of CdTe samples and drafted the manuscript.
RM and WAAM worked on XPS measurements and preparation of the manuscript. KB helped
with TEM and the characterization of the samples. PR and MAS participated in the preparation,
revision, and finalization of the manuscript. All authors read and approved the final
manuscript.

Acknowledgments

This work was supported by CAPES, CNEN, CNPq, and FAPEMIG. The authors would like
to acknowledge the Brazilian Nanotechnology National Laboratory LNNano and LNLS for
providing the equipment and technical support for the experiments involving transmission
electron microscopy. We thank D.L. Ferreira and P. Licínio for the support and the
use of the DLS.